Recently, systems using frequencies in a band of 800-1000 MHz have begun to be largely used in mobile communication such as with automobile telephones as a local oscillator for the communication apparatus. These frequencies have also been used for CATV of a television as a fixed local oscillator of a tuner of a double-super system suitable for multi-channel receiving, and utilization of a coaxial resonator using a dielectric ceramic is also being considered for such frequencies.
In the prior art, there is an apparatus which is elucidated, for example, in "National Technical Report," Vol. 31, No. 2, April, 1985. Hereinafter, referring to the figures, this local oscillation apparatus of the prior art will be elucidated.
FIG. 1 shows a local oscillation apparatus using the coaxial resonator of the prior art. Referring to FIG. 1, numeral 41 designates the coaxial resonator using dielectric ceramics, numeral 42 designates a coupling capacitor between the coaxial resonator 41 and an oscillation circuit, numeral 43 designates a coupling capacitor between the coaxial resonator 41 and a variable capacitance diode 44, numeral 44 designates the variable capacitance diode for varying oscillating frequency, numeral 45 designates a transistor for oscillation, numerals 46, 47 and 49 designate capacitors for feedback, numeral 48 designates a capacitor for grounding the base of the transistor 45, numeral 50 designates a bypass capacitor, numerals 51-54 designate resistors for supplying a power source and for giving a bias to the oscillating transistor 45, and numeral 55 designates a resistor for applying a voltage to the variable capacitance diode 44.
As to the local oscillation apparatus which is composed as mentioned above, the operation is elucidated hereafter.
The prior art oscillator has a constitution of a Clapp oscillation circuit of grounded base type. The coaxial resonator 41 is a coaxial resonator of the .lambda./4 type, and the inductive property (L component) in the vicinity of a resonance point is utilized. FIG. 2 shows a basic circuit of the Clapp oscillation circuit type, and its oscillation frequency is shown as follows. ##EQU1## Further, the variable capacitance diode 44 is connected in parallel to the coaxial resonator 41. By a control voltage applied to a terminal A, a capacitance of the variable capacitance diode 44 is varied, and the resonance frequency of the coaxial resonator 41 is equivalently varied. The oscillation output signal is taken out of a terminal B.
However, in the constitution mentioned above, as shown in the equation (1), as elements for deciding an oscillation frequency, there are utilized the capacitors 42, 46, 47 and 49 of FIG. 1. Capacitances of these capacitors are approximately 4 pF-5 pF, and a variation of oscillation frequency due to a temperature change is greatly influenced by temperature characteristics of these capacitors. That is to say, the coaxial resonator 41 output changes by about 10 ppm/.degree.C. and has an almost linear variation, but as to the whole oscillation circuit including the above-mentioned capacitors, as shown in FIG. 3, it shows a curve with a width W of several hundred KHz. Therefore, even if temperature compensation is accomplished, a frequency error corresponding to the width arises yet, and there is problem that contriving a complete temperature compensation is difficult. Furthermore, since the circuit is made with the above-mentioned plural capacitors, there is a problem that making the oscillator as an integrated circuit is difficult.